Recent versions of copying apparatus have various extra capabilities including those for a composite mode of operation and a duplex mode of operation. As well known in the art, a composite mode of copying operation is used for the purpose that images within two different specified areas of a document or images on different document sheets are duplicated in two successive cycles of operation on a single face of a single copying sheet. In a duplex mode of copying operation, images within two different specified areas of a document or images on different document sheets is duplicated in two successive cycles of operation on the opposite faces, respectively, of a single copying sheet.
During a composite or duplex mode of copying operation, a copying sheet having reproduced images already printed on one side thereof at the image reproducing stage of the apparatus is, after being turned out or not turned out passed to a paper recirculation unit for temporary storage therein. From the paper recirculation unit, the copying sheet is for a second time supplied to the image reproducing stage so that reproduced images are further printed on the previously printed face of the sheet or on the reverse face of the sheet.
In the process of such copying operation, it may happen that two or even more copying sheets are fed to the image reproducing stage of the apparatus during the earlier or "first" half cycle of the two consecutive cycles of composite or duplex mode of operation. Reproduced images are printed on one face of one these two or more copying sheets, which are then passed over to the paper recirculation unit. It is, in this instance, likely that, among the copying sheets thus stored temporarily, only one of them is supplied from the paper recirculation unit during the later or "second" half cycle of the composite or duplex mode of operation. The result that the remaining one or more copying sheets, which may all consist of plain or non-printed sheets or may include a single printed sheet, are left in the paper recirculation unit without being discharged out of the paper recirculation unit.
There may also be a case where two or even more of the copying sheets passed to and stored in the paper recirculation unit are picked up and recirculated from the paper recirculation unit during the "second" half cycle of the composite or duplex mode of operation.
An occurrence of such a "double feed" of copying sheets would result in failure in executing a composite or duplex mode of operation. To preclude an occurrence of double feed of copying sheets, an image duplicating apparatus using a double feed sensor to detect a stack of two or more copying sheets from a beam of light reflected from or transmitted through the stack of paper. An example of the sensor using reflected light is taught in Japanese Provisional Patent Publication (Kokai) No. 58-07655 and an example of the sensor using reflected light is taught in Japanese Provisional Patent Publication (Kokai) No. 58-31353. The double feed sensor of the former type is arranged to detect an occurrence of double feed in respect of the copying sheets to be fed to the image reproducing stage of the apparatus and is for this reason useless for the detection of double feed from the paper recirculation unit. The double feed sensor of the latter type has a drawback in that the signal resulting from the light transmitted through a sheet or sheets of paper could not discriminate between printed and non-printed sheets.
The present invention contemplates the provision of an improved image duplicating apparatus which will overcome these difficulties thus far experienced in a prior-art image duplicating apparatus the type having composite and duplex modes of operation.
It is accordingly a prime object of the present invention to provide an image duplicating apparatus which features means operative to detect if the number of printed outputs produced by the end of the copying operation for a given document sheet equals the total number of prints initially selected and entered by the operator for composite and duplex modes of operation.